Field of the Invention
The present invention relates to a porous separator for a fuel cell and, more particularly, to a porous separator for a fuel cell in which the porous separator has a modified passage aperture, thereby minimizing destruction of a gas diffusion layer or membrane electrode assembly, the destruction being attributable to stress concentration.
Description of the Related Art
A fuel cell stack for a vehicle includes a plurality of cells connected to each other. Fuel and coolant are supplied from one side of the fuel cell stack and discharged from the opposite side. In each cell of the fuel cell stack, separators are arranged on respective principal surfaces of a membrane electrode assembly (MEA) covered by gas diffusion layers. Cells having the above-described structure are stacked in series to form a fuel cell stack.
Further, an MEA is an electrolyte membrane covered by an air electrode and a fuel electrode allowing a reaction between hydrogen and oxygen to occur. The MEA is disposed in the center of a cell of a fuel cell stack. Gas diffusion layers are stacked on outer surfaces of the air electrode and the fuel electrode. In addition, separators, each with a flow field that allows fuel to be introduced into the cell through it or allows water produced through a hydrogen-oxygen reaction to be discharged through it, are arranged extraneous to the respective gas diffusion layers in which gaskets are interposed between the gas diffusion layers and the separators.
The separator is typically structured such that lands that are in tight contact with the gas diffusion layer and channels (flow fields) that serve as a flow channel of a fluid are arranged alternately. Since the lands and channels (flow fields) are arranged alternatively and are both meandering, the channels in one surface of a separator that faces the gas diffusion layer are used to allow a reaction gas such as hydrogen or air to pass therethrough and the channels in the opposite surface of the separator are used to allow coolant to pass therethrough.
In a fuel cell stack, an oxidation reaction of hydrogen progresses in a fuel electrode which produces protons and electrons. The produced hydrogen ions and electrons move to an air electrode through an electrolyte membrane and a separator. Thus, the hydrogen ions and electrons that are supplied from the fuel electrode and oxygen existing in air undergo an electrochemical reaction in an air electrode. This reaction produces water and electric energy, and at this time the flow of electrons produces electrical energy.
According to this structure, since channels serving as flow field for reaction and gas and coolant are formed by stacking a separator for an anode and a separator for a cathode on each other, the structure of a unit cell of a fuel cell stack is simplified. However, the unevenness of the surface of the separators attributable to channels and lands formed in the separators leads to a lack of uniformity in surface pressure, which results in an increase in electric resistance and immoderate stress concentration around the lands. This results in destruction of a gas diffusion layer and deterioration of diffusivity of a reaction gas.
When a structure with fine pores such as metal/carbon foam or wire mesh is inserted into a reaction surface instead of a conventional flow field having a channel shape, movement of a reaction gas and water becomes easier. Furthermore, a gas diffusion layer is uniformly compressed to distribute surface pressure, thereby minimizing electric resistance and maximizing performance of a fuel cell. However, existing flow field structures with fine pores have disadvantages of high manufacturing cost and increased weight and volume compared to conventional flow field structures. Therefore, mass-productivity of the flow field structures is deteriorated.
FIG. 1 illustrates a conventional porous separator according to the related art. The conventional porous separator has a plurality of passage apertures 43 formed in inclined surfaces (e.g., on a side sloped surface thereof) of a flow field plate 40. The passage apertures are arranged at regular intervals in a longitudinal direction (e.g., gas flow direction) of the flow field plate 40. Since concave and convex structures are repeatedly arranged in the longitudinal direction, fuel may be smoothly diffused in a reaction surface of the flow field plate 40. Particularly, since a flow rate of a reaction gas greatly increases under high current conditions in which fuel consumption is high, flow resistance increases due to the concave and convex structures in a porous plate. In other words, the effect of the porous plate is maximized.
Furthermore, a reaction gas that passes through a passage aperture formed in a one-side inclined surface of the flow field plate is stopped by the opposite-side inclined surface (e.g., the slopes are of opposite inclines or angles). Therefore, the reaction gas flows in the widthwise direction of the flow field plate to move to an adjacent aperture in a next channel. Since the reaction gas flows along a zigzag path, diffusivity of the reaction gas is increased. A micro porous flow field is disposed to be adjacent to a gas diffusion layer in a cell. A reaction gas that is introduced through the porous flow field passes through the gas diffusion layer and reaches an MEA whereby a reaction occurs.
The gas diffusion layer is a micro porous layer that is a conglomerate of carbon fiber. To facilitate diffusion of a reaction gas and discharge of water produced through a chemical reaction, it is necessary to minimize destruction of a micro porous structure in a gas diffusion layer, the destruction being attributable to coupling force of cells of a fuel cell stack. On the other hand, since the micro porous flow field is also used as a transmission path for electricity generated through a chemical reaction as well as a flow path for a reaction gas, it is desirable to minimize contact resistance in an interface surface by increasing the coupling force of cells of a fuel cell stack.
However, as illustrated in FIGS. 2 and 3, due to a structural characteristic of a porous body in which passage apertures are formed in inclined surfaces, a cut portion of a passage aperture 43 is in contact with the gas diffusion layer (GDL) and stress is concentrated at the contact area. For this reason, carbon fiber that forms the gas diffusion layer is destroyed. In other words, since the cut surface of a passage aperture comes into tight contact with the gas diffusion layer due to the structural characteristic of a porous body and the tight contact causes stress concentration, excessive destruction of the gas diffusion layer occurs. Thus, deterioration is caused in diffusivity of a reaction gas and water discharge performance and physical damage to a membrane electrode assembly.
The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.